After doing some more experimenting in the beta looks like this is how it's done...
Each infantry based Map has it's own set of four heroes (one for each side of the four factions).
Utapau has Obi-Wan (Republic) & Grevious (CIS) / Han Solo (Rebels) & Boba Fett (Empire); Tantive IV has Yoda (Republic) & Grevious (CIS) / Princess Leia (Rebels) & Darth Vader (Empire)
Now in SWBF1 the Heroes were all generic Jedi. Now the Jedi actually have two types of swings. Their standard combo and a "backwards attack" when you're moving backwards. They "float" when they're sprinting and move very fast (although Grevious strides and gets in his own way, and if you run into a wall you're basically stuck in it for awhile). Vader looks a little scarier this time around and WALKS everywhere, so he's powerful but very slow (you have to sprint or Jedi Spirit to get him anywhere really).
Their "Jedi Spirit" power is apparently the same as sprinting, but you can float up a little bit (less than normal jump height though). Force Jump is just a jump at the height of your jump and you jump about twice as high and a little floatier.
Each Jedi has to cycle "powers".. they all have Saber Throw plus one more (Yoda & Obi-Wan: Push, Vader: Choke). Grevious just has a "buff" power that's infinite. Edit: Correction, Yoda has "Push" and "Pull" which doesn't seem to work very well (Yoda's best advantage is his small size and speed!).
The "buff" powers are a ground based thing that affects people in your area and gives them a temporary boost like invulnerability, extra armor or extra damage, etc.
The Hero has a power bar that slowly decreases, when it's gone they're gone. If you get hit, it takes some off your bar, so you can die in 2 seconds if you're not careful. If you KILL somebody you get a little bit added to your bar (no clue what killing teammates does, but it ought to take off your bar IMHO!). So theoretically you could stay as a Hero forever if you just kept killing... though each kill only adds a tiny bit.
As far as Jedi are concerned, their "force bar" is identical with their normal energy bar (the "tired meter" in this game which I think is a great idea). Force Jump, Force Powers, sprinting, Jedi Spirit, rolling... it all takes your energy away and you have to then let it recharge. "Blocking" is a toggle, which vanishes if you roll or jump or attack. It seems to block everything in front of you (except explosives) but it drains your energy bar at a constant rate (regardless of if you are actually blocking anything or not). So it is of limited usefulness (I guess it might be good against a chaingunner).
I'm pretty sure I'm using the default settings here (on my test server).
So anyway, so far at least if the beta is any indication, the Heroes seem pretty good. And giving the server the ability to customize them should cut down on problems, though I imagine some servers might make it a bit too easy and then people would just be them all the time... or too many purists will turn them off out of fear (remembering SWBF1) and miss out on something fun.